Removing fan energy from rated COP

asked 2024-01-19 13:19:31 -0500

willyJohan's avatar

updated 2024-01-23 14:10:27 -0500

This has been covered in a few posts here before, but I'm not sure the prototype approach or the 90.1 budget system formulas will give me what I'm looking for (representing generic systems with a specific rated COP based on default PDs, fan efficiencies, autosized capacities and airflows)

To calculate COP-nf for a generic system, does the following approach (basically removing calculated fan power from the denominator of the COP) work?

  • total power input [BTU/hr.] = capacity / COP
  • fan power input [BHP] = cfm x PD / (fan-eff x 6356)
  • compressor power input = total power in - fan power in
  • COP-nf = capacity / compressor power input

(applying the appropriate conversions)

EDIT: Thinking a bit more about the DOE Prototype method, I'm realizing I should probably add the fan heat to the cooling capacity (i.e., the numerator) for COP-cooling-nf and subtract it for COP-heating-nf to account for the impact fan heat.

and, if this is a valid approach, for VRF/multi-split systems, is the supply fan power merely the sum of each connected indoor unit's fan power?

lastly, what about other elements of systems that consume energy, could/should these also be removed from the COP denominator? Including those that are defined within the object where COP-nf is an input, such as:

for various coil types

  • Crankcase Heater Capacity
  • Resistive Defrost Heater Capacity

or other objects necessary to define a system (but don't take COP-nf as an input) such as


  • Parasitic Electric Energy Use

For some background if someone comes across this post and would like to know about the standard approaches:

DOE Prototype approach:

cop = (eer / 3.413 + R) / (1 - R)

where R is "the ratio of supply fan power to total equipment power at the rating condition." and 0.12 is typically used

and here are the various formulas from 90.1 (12.5.2.c in 2022):

  • cop = -0.0076 * seer * seer + 0.3796 * seer
  • cop = 1.48E-7 * coph47 * Q+ 1.062 * coph47 (system 6 and 9 heating)
  • cop = -0.0296 * hspf * hspf + 0.7134 * hspf
  • cop = 7.84E-8 * eer * Q+ 0.338 * eer
  • cop = 0.3322 * eer - 0.2145 (system 8 and 10 cooling)
  • cop = 1.1329 * cop - 0.214 (system 8 heating)
edit retag flag offensive close merge delete


@mdahlhausen (tagging you as i see your ID associated with the file), it looks like the openstudio-standards file Prototype.utilities.rb has the prototype formula above defined as giving the COP with fans. Am I incorrect in thinking it should be the opposite?

willyJohan's avatar willyJohan  ( 2024-01-20 13:38:22 -0500 )edit

Short answer is yet. @Jeremy has some fixes in to change it for the next release.

mdahlhausen's avatar mdahlhausen  ( 2024-01-30 14:59:30 -0500 )edit